The invention is related generally to antenna systems and, more particularly, to rotating directive-beam antennas with polarization control.
In many applications it is desirable to provide an antenna system capable of scanning a beam 360.degree. in azimuth, e.g., a horizon scan. In many such applications, a rotatable antenna system is employed. Many rotatable antenna systems utilize an RF rotary joint wherein the RF feed is rotated along with the antenna. RF rotary joints have been known to be unreliable especially where the rotational speed of the antenna is substantial and where extended periods of continuous use are required. Also, rotary joints are difficult to manufacture for operation at millimeter wave frequencies.
Some antenna systems circumvent the need for an RF rotary joint by fixing the feed in place while rotating a reflector about the feed axis to provide the necessary scanning. A limitation of such systems has been that they do not provide a fixed linear polarized beam throughout the scan. As the feed remains stationary and the reflector rotates about the feed axis, the orientation of polarization varies by 90.degree. during each 90.degree. of rotation of the reflector. For example, the polarization may change from horizontal to vertical in the 90.degree. of scan. Thus, for each revolution of the reflector, the polarization alternates between vertical and horizontal twice. If the feed is not circularly polarized, no energy will be received for orthogonal linear polarizations. If the feed is circularly polarized, there will be a 3 db loss of energy for linear polarizations and a complete loss if the received energy is of the opposite sense of polarization from that of the feed. If an orthomode transducer is employed at the fixed feed to capture a fixed linear polarization, the energy will switch between the ports of the transducer in dependence upon the position of the reflector. Thus, further complexities are involved in applying a switching circuit at the outputs of the transducer to conduct the desired polarization to the processor.
One method for retaining the same polarization throughout the scan is to use multiple feeds with a rotating reflector. Such a method is shown in M. I. Skolnik, INTRODUCTION TO RADAR SYSTEMS, 2ed., McGraw-Hill, 1980 pgs 243-244. However, such a system requires more complexity than the single fed system, including the timing for energizing the feeds, and has a relatively large physical size and weight.
In most applications it is desirable to have an antenna system which has the same polarization as a particular target throughout its scan. For maximum received signal strength, the receive antenna should be polarized in the same manner as the signal to be received. Where the orientations of linear polarization are different, the extracted energy is reduced in proportion to the cosine of the relative angle between them. Where a circularly polarized feed is used, a loss of 3 dB is incurred due to polarization mismatch. This loss of 3 dB is significant in some applications.
Accordingly, it is desirable to provide a rotatable antenna system which avoids the problems associated with a rotary joint, which can function efficiently at millimeter wave frequencies, and which has a fixed linear polarization throughout its 360.degree. scan.